Re: Freeze avoidance of very large table.

> I would also like to see the visibilitymap_test function exposed in SQL,
> so we can write code to examine the map contents for particular ctids.
> By doing that we can then write a formal test that shows the evolution of tuples from insertion,
> vacuuming and freezing, testing the map has been set correctly at each stage.
> I guess that needs to be done as an isolationtest so we have an observer that contrains the xmin in various ways.
> In light of multixact bugs, any code that changes the on-disk tuple metadata needs formal tests.

Attached patch adds a few function to contrib/pg_freespacemap to
explore the inside of visibility map, which I used for my test.
I hope it helps for testing this feature.